1. Field of the Invention
The present invention is directed to a method and apparatus for proof testing a brittle material, and in particular proof testing a glass or glass ceramic sheet.
2. Description of Related Art
Display devices utilizing plasma, liquid crystal, or organic light emitting diode display elements, to name a few, are fast overcoming cathode ray tube (CRT) displays in commercial products, finding use in a myriad of applications, from cell phones to televisions. However, the introduction of very thin, flexible displays is only in its infancy. This is due in no small part to the tremendous structural demands placed on such display devices: they must be capable of withstanding repeated flexing or bending without harm to the device or the substrate on which it is disposed; due to the intended use of flexible displays in portable devices, they are expected to withstand rough handling, again without undue harm to the device or substrate, and; they should be capable of withstanding a bending radius that can be less than 2 cm, and less than 1 cm in some cases.
One material contemplated for use in flexible displays is glass. Glass is generally chemically resistant, transparent, can form a hermetic barrier or seal, and may be formed into very thin sheets. Sheets in excess of 10 m2 having thicknesses less than 1 mm, and even less than 0.7 mm have been produced and routinely used, and glass sheets are soon expected to reach dimension of at least about 100 m2. In a typical display manufacturing process, multiple displays are formed using one or more large glass sheets or substrates. The displays are then separated into individual display units, usually by scoring and breaking. Thus, very large glass sheets are efficiently utilized by producing as many display units as possible.
Cutting glass, and in this case glass sheets, generally forms flaws (e.g. cracks) in the edges of the glass sheets. These flaws can serve as fracture sources, and thereby reduce the strength of the sheets, particularly if the glass is flexed such that the flaw experiences tensile stress. Generally, typical display devices do not experience significant flexing, thus the existence of these flaws is of less concern.
Flexible displays, by the very nature of their flexibility, may produce significant stress in the display substrate(s), either during the manufacturing process or in use. Thus, flaws that might be present in the glass may experience stresses sufficiently great that the glass will crack. Since typical display manufacturing involves cutting the glass to form individual displays, and cutting is known to create multiple flaws in the glass along the cut edge, this bodes poorly for the fate of glass substrate-based flexible display devices.
Attempts to mitigate flaws at the edges of glass sheets have included laser cutting, grinding, polishing and so forth, all in the attempt to remove or minimize the flaws that are created when the glass sheet is cut to size. However, many of these approaches are unsatisfactory for flexible display application, either because the technique is incapable of removing flaws down to the size needed for the expected stresses, or the technique is difficult to apply to such thin glass sheets (less than about 0.4 mm thick). Acid etching of the glass edges may be used, but may also degrade the display device disposed on the substrate. Thus, it would appear that regardless the method of cutting used, flaws will continue to be formed in glass sheets, and in particular at the edges of the sheet.